Microstructural sensitivity and deformation micro-mechanisms of a bimodal metastable β titanium Ti–7Mo–3Nb–3Cr–3Al alloy

Abstract

The bimodal microstructure of a metastable β Ti–7Mo–3Nb–3Cr–3Al (Ti-7333) alloy was successfully tuned through various thermomechanical processing steps, to achieve strength and ductility combinations, in the range between 903 and 1443 MPa and 2.9 % and 17 %, respectively. The influence of the microstructural variables on the deformation micro-mechanisms and fracture mechanisms were further elucidated by the assistance of TEM and fractographic analysis. The results indicate that the strength of bimodal Ti-7333 is mainly governed by secondary αS precipitation through the introduction of a high density of α/β interfaces that effectively block dislocation transmission, whereas ductility is tuned through the control of the primary αP, rod-like αr and retained β phases. Moreover, ductility can be extremely deteriorated if the formation of a continuous grain boundary αCGB phase is not prevented. Plastic deformation of bimodal Ti-7333 is dominated by dislocation slip and tangling, whereas the microstructural sensitivity that accelerated at high strength level is originated from the deformation incompatibility between the fine acicular αS and parent β. In addition, the {10–11}<-1012>α micro-twinning of equiaxed αP phases also contributes to improved ductility and coordinates deformation to an extent. The fractographic analysis revealed either fracture by microvoid coalescence or a mixture of intergranular fracture and microvoid coalescence, depending on thermomechanical condition. The mixed fracture mode results in a relatively flat crack propagation path and reduced ductility; whereas a tortuous crack propagation is obtained when the alloy solely undergoes fracture by microvoid coalescence.